1. Field of the Invention
The present invention relates to the general field of intrusion detection and monitoring circuitry which monitors the opening of a door. In particular, the present invention relates to such a system for monitoring the gasket resistance of a door of an outdoor electrical equipment cabinet to determine unauthorized access to the cabinet.
2. Description of the Related Art
Due to the recent explosion of the cellular industry, there is an ever increasing need for security in cellular systems. Cellular base stations often include equipment which is located outdoors. This equipment is typically stored in metal cabinets sealed by hinged doors. These cabinets are usually locked, but it is not impossible for individuals to gain access to the contents of these cabinets without the knowledge of the cellular service provider. In order to monitor these cabinets, there are currently provided intrusion detection systems. These intrusion detection systems are usually monitored from a remote location, such as a cellular switching station.
The typical intrusion detection systems for detecting door opening currently in use include a plunger and microswitch arrangement. A plunger and microswitch are attached to the door or portion of the cabinet at a location near the hinges, so that when the cabinet door is opened the plunger is no longer depressed against the microswitch, and the microswitch is turned on (using a normally "on" type microswitch). The "on" condition of the microswitch is typically wired into either a landline or a cellular radio contained within the cabinet. In the wireless cellular embodiment, an associated antenna sends a signal from the cellular base station to the cellular switching station or other remote location, identifying that the door has been opened. When the door is closed the plunger engages the microswitch and turns the switch off, and hence, no signal is transmitted via the radio and antenna to the cellular switching station. By this method, the operator at a cellular switching station can determine if there has been unauthorized access to the cellular equipment located at the base station.
Most outdoor telecommunications cabinets also include an electromagnetic interference (EMI) gasket lining the space between the door and the cabinet. These gaskets serve to limit the amount of EMI entering into and escaping from the telecommunications cabinet. Indeed, the Federal Communications Commission (FCC) has certain requirements on EMI levels, and these gaskets meet those requirements. Because the gasket is conductive, it also provides electrical continuity between the door and the cabinet.
The typical microswitch-plunger intrusion detection systems are deficient in that they cannot detect tampering with the door if the door has not been fully opened. For instance, one could pry the upper or lower portion of the door away from the cabinet without disturbing the microswitch and plunger. Further, traditional intrusion detection systems cannot measure the integrity of the EMI door gasket. Since the gasket is fashioned to prevent leakage of electromagnetic interference, monitoring of the integrity of the gasket is also useful in determining the amount of EMI leakage.